A previously proposed electromagnetic clutch includes a rotor, a stator and an electromagnetic coil. The electromagnetic coil is received in an inside space of the stator. The stator is placed in an inside space of the rotor while the electromagnetic coil is received in the stator.
More specifically, the rotor has a double cylindrical tubular structure that includes an outer cylindrical tubular portion and an inner cylindrical tubular portion. A space, which is formed between an inner peripheral surface of the outer cylindrical tubular portion and an outer peripheral surface of the inner cylindrical tubular portion, is the inside space of the rotor. Since the rotor is rotated about the stator, the inner peripheral surface of the outer cylindrical tubular portion and the outer peripheral surface of the inner cylindrical tubular portion of the rotor are both in parallel with an axial direction. Therefore, a distance between the outer cylindrical tubular portion and the inner cylindrical tubular portion of the rotor is constant.
Also, the stator has a double cylindrical tubular structure that includes an outer cylindrical tubular portion and an inner cylindrical tubular portion. A space, which is formed between an inner peripheral surface of the outer cylindrical tubular portion and an outer peripheral surface of the inner cylindrical tubular portion of the stator is the inside space of the stator. The outer peripheral surface of the outer cylindrical tubular portion of the stator is opposed to the inner peripheral surface of the outer cylindrical tubular portion of the rotor, and the inner peripheral surface of the inner cylindrical tubular portion of the stator is opposed to the outer peripheral surface of the inner cylindrical tubular portion of the rotor. Therefore, similar to the inner peripheral surface of the outer cylindrical tubular portion and the outer peripheral surface of the inner cylindrical tubular portion of the rotor, the outer peripheral surface of the outer cylindrical tubular portion and the inner peripheral surface of the inner cylindrical tubular portion of the stator are both in parallel with the axial direction, and thereby a distance between the outer peripheral surface of the outer cylindrical tubular portion and the inner peripheral surface of the inner cylindrical tubular portion of the stator measured in the radial direction of the stator is constant (see, for example, the patent literature 1).
With respect to the electromagnetic clutch, there are customer demands for reducing the size and the weight of the entire electromagnetic clutch, improving the transmission torque and reducing the electric power consumption. In order to meet these customer demands, it is necessary to increase a coil space without deteriorating the magnetic performance for the previously proposed electromagnetic clutch. Here, the coil space refers to an installation space for installing the coil.
For example, when the coil space is increased in comparison to the previously proposed electromagnetic clutch while maintaining the size of the rotor, the transmission torque can be improved. Furthermore, when the size of the rotor is reduced in comparison to the previously proposed electromagnetic clutch while maintaining the size of the coil space, the size and the weight of the entire electromagnetic clutch can be reduced.